The invention relates to a ventilation arrangement for the fuel tank of a vehicle with valves allocated to the head space thereof for producing a maximum pressure, which is not to be exceeded, a minimum pressure below which the pressure must not fall, and a randomly adjustable pressure which is intended for the tank filling process.
During the development of the drive means of motor vehicles, vehicles have been developed which, in addition to the conventional internal combustion engine, also contain an electromotive drive fed by a battery. These vehicles are also designated hybrid vehicles and make it possible to travel for a certain time using only an electric motor.
The mineral oil-based fuel required for operation of the internal combustion engine is stored in a fuel tank from which, when the engine is not running, vapours escape owing to vapour pressure differences with respect to the external atmosphere, these vapours passing through an activated carbon filter inserted into a ventilation line, the filter being intended to adsorptively bind hydrocarbon components. When the engine is running, on the other hand, these retained hydrocarbon components are picked up by air flowing in and supplied to the combustion process of the engine so that, either way, they are prevented from escaping into the environment.
Since, during exclusively electric operation of the vehicle, no external air flows through the activated carbon filter, stored hydrocarbon components are regenerated during this time. In order still to achieve at least almost complete regeneration despite the fact that the internal combustion engine is thus operating for a reduced period of time, a higher pressure than the external pressure is required in the head space of the fuel tank in order to reduce the quantity of hydrocarbon components to be adsorbed in the activated carbon filter. It is generally known to limit the pressure in the head space of the fuel tank for safety reasons by means of a pressure relief valve. When excess pressure or a maximum pressure occurs, this produces a through connection between the head space and the external atmosphere via a ventilation line. It is further known to limit the pressure drop within the tank during operation of the internal combustion engine as a result of continuous removal of fuel by means of a valve which produces a connection to the external atmosphere when the pressure falls below a minimum pressure, thereby achieving a pressure equalisation.
When the vehicle's tank is being filled, the gas volume displaced by the inflowing liquid fuel as well as fuel vapours must be channelled out of the tank. In so doing, a pressure equalisation with the environment must be produced by complete ventilation, since otherwise fuel will exit the filling pipe of the tank.
However, this requirement is contrary to the desire of having increased pressure in the fuel tank.
Operation of a fuel tank in which an increased internal pressure must be maintained therefore requires the presence of three valves. These must have sufficient sealing tightness or otherwise the pressure being created in the tank would be reduced if the vehicle is stationary for a long period and the initially mentioned conditions of desorption and adsorption, balanced according to quantity, of hydrocarbon components in the activated carbon filter could not be fulfilled.
It is known, to use respective spring loaded individual valves in order to limit a maximum pressure which is not to be exceeded and a minimum pressure below which the pressure must not fall. In addition to assembly costs, a problem with the use of three separate valves, including one intended for ventilation during tank filling, is, however, the space for housing such components which is extremely limited in precisely these hybrid vehicles owing to the presence of the electromotive drive.